This invention relates generally to liquid-cooled armature bar clip-to-strand connections and more particularly, to a clip configuration and braising method for braising generator armature winding bars to hydraulic header clips.
The armature windings on large steam-turbine generators are generally water-cooled. The armature windings comprise an arrangement of half coils or armature bars (collectively referred to as “armature bars” or “bars”) connected at each end through copper or stainless steel fittings and water-cooled connections to form continuous hydraulic winding circuits.
Water-cooled armature winding bars are comprised of a plurality of small rectangular solid and hollow copper strands arranged to form a bar. The rectangular copper strands are generally arranged in rectangular bundles. The hollow strands each have an internal duct for conducting coolant through the bar. The ends of the strands are each brazed to a respective hydraulic header clip. The hydraulic header clip serves as both an electrical and a cooling flow connection for the armature winding bar.
The hydraulic header clip is a hollow connector that includes an enclosed chamber for ingress or egress of a cooling liquid, typically deionized water. At one open end, the clip encloses the ends of the copper strands of the armature winding bar. A braze alloy bonds the end sections of the strands to each other and to the hydraulic header clip. The braze joints between adjacent strand ends and between the strand ends and the clip should retain hydraulic and electrical integrity for the expected lifetime of the winding. A typical life time of a winding is on the order of tens of years.
Internal surfaces of the brazed joints between the clip and the ends of the strands are constantly exposed to the deionized, oxygenated water flowing through the clip and the hollow strands. The exposure of the brazed surfaces to the coolant can result in corrosion of the armature winding bar and hydraulic header clip. Corrosion tends to occur in the crevices of the joints between the hydraulic header clip and the strand ends of the armature bar, and in the crevices between the strand ends. Corrosion of a phosphorous-containing braze alloy and adjoining copper strand surfaces can occur if critical crevice geometry and crevice water chemistry conditions are present. If allowed to progress through a joint, corrosion will eventually result in a water leak through the entire effective braze joint length and compromise the hydraulic integrity of the clip-to-strand joint. Accordingly, there is a long felt need for a corrosion-resistant clip-to-strand braze joint. The benefits of crevice corrosion-resistant braze joint are expected to include improved generator availability and generator reliability.
U.S. Pat. No. 5,796,189 discloses an arrangement where all of the strands are cut to the same length and the copper-phosphorous (BCuP) braze alloy is pre-placed flush to the ends of the strands. A braze alloy anti-wetting agent is used on the ends of the hollow strands to prevent plugging of the hollow strands and an inert purge gas is used during the brazing cycle. Use of the anti-wetting agent, although effective for preventing hollow strand plugging, can result in strand faying surface contamination and a poor effective braze joint length.
U.S. Pat. No. 6,784,573 teaches the use of extended hollow strands in combination with the use of a de-oxidizing gas as the purge gas. The extended hollow strands eliminate the need to use an anti-wetting agent on the ends of the hollow strands and the de-oxidizing purge gas limits oxidation during the brazing cycle, de-oxidizes strand surfaces and the braze alloy prior to alloy melting, and improves braze alloy wetting and flow. However, this practice continues the use of a phosphorous-containing braze alloy and, therefore, the potential for crevice corrosion still exists, although it is greatly minimized due to the significant reduction, or the elimination in most cases, of surface initiation sites.
A recent and commonly owned patent application Ser. No. 10/841,964, filed May 10, 2004, teaches a brazed joint between an armature bar strand package and an end fitting including: a plurality of strands arranged in a tiered array and forming the strand package; a cavity in the end fitting, free ends of the plurality of strands extending through the opening and received in the cavity; and an essentially phosphorous-free copper-silver braze alloy joining the free ends of the plurality of strands to each other and to interior surfaces of the end fitting.
A recent and commonly owned patent application Ser. No. 10/991,371, filed Nov. 19, 2004 teaches a brazed joint between an armature winding bar strand package and a hydraulic header clip end fitting including: a plurality of solid strands and a plurality of hollow strands arranged in a tiered array and forming the strand package, the plurality of hollow strands having free ends that extend axially beyond corresponding free ends of solid strands; a cavity in the end fitting into which the free ends of the hollow strands and solid strands extend, and an essentially phosphorous-free silver braze alloy joining the free ends of the plurality of hollow strands and said corresponding free ends of solid strands to each other and to interior surfaces of the end fitting, wherein said braze alloy forms an isolation layer over the free ends of the solid strands.